32 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



15.5 percent of the 253,548 fish produced, the un- 

 observed mortality (difference between net pro- 

 duction and observed distribution and losses) was 

 much higher, amounting to an estimated 42,900 

 fish or about 17 percent of the total production. 

 The causes for these unobserved losses were dif- 

 ficult to assess, but presumably can be attributed 

 to a variety of factors: vandals entering the plant 

 and removing unknown quantities of fish; loss of 

 fish through the drain; predation by dragonfly 

 nymphs; and canniljalism. Davis (1946: p. 9) in 

 reporting on "uncounted mortality" in trout 

 hatcheries stated that this was due "either to im- 

 proper construction of the raceways, so that many 

 of the fish are able to escape; to the attacks of 

 enemies, such as fish-eating birds; or to 

 cannibalism." 



At the Kewalo plant, observed mortalities 

 among the young amounted to 44,600 fish or about 

 13 percent of the .347,700 fish produced. 



DISEASE PREVENTION AND CONTROL 



Prevention and control of fish diseases and in- 

 festation of parasites are important factore in the 

 success of any type of fish-rearing program. Fish, 

 like other animals, are subject to a wide variety 

 of infectious diseases and parasites and seem par- 

 ticularly susceptible in an unnatui-al or artificial 

 environment. 



Several outbreaks of disease or parasitic infesta- 

 tion were observed among tilapia at both the pilot 

 plant and the Kewalo plant. All dead fish were 

 examined microscopically for signs of unnatural 

 conditions, such as mucous film or irregular 

 blotches on the body and unusual blisters or 

 swelling. When sick fish were observed in the 

 tanks, similar examinations were made on their 

 external surfaces, gill region and, on occasion, 

 the gastrointestinal tract. It was not always pos- 

 sible to distinguish between losses from diseases 

 and from other causes, although in many instances 

 of high mortality rates parasites were seen and 

 identified to genus. Several exceptions occurred 

 when the cause of high mortality rates among the 

 fry could not be determined by isolation of any 

 causative organism. By careful observation of 

 symptoms, it was possible on a few occasions to 

 restrict the probable cause of death to a virus 

 infection. In general, the fry, juveniles, and 

 adults were susceptible to infection in that order. 



Trichodina spp. 



Infestation by the ectoparastic trichodinids, con- 

 sidei'ed one of the most highly specialized proto- 

 zoans, was the most common malady among adult 

 and young tilapia, with infestation along the 

 dorsal fins, dorsal region of the caudal peduncle, 

 and gill region being most prevalent. Tilapia 

 with trichodiniasis were sluggish, showed loss of 

 appetite, had a reddish tinge on the skin in the 

 caudal-peduncle region, and, in some instances, in 

 and around the region of the dorsal fins, they had 

 white blotches accompanied by a fraying of these 

 fins. 



According to Davis (1953: p. 220), Tnchodina 

 is very easy to control. We used several recom- 

 mended treatments such as salt (1.23 percent), 

 acetic acid (1 : 500), formalin (1 : 4,000), pyridyl- 

 mercuric acetate or PMA (2 p.p.m.), copjier sul- 

 fate (0.5 p.p.m.), and potassium permanganate 

 (3 p.p.m.) . All of these reagents proved effective 

 in controlling the disease; however, as more ex- 

 perience was gained, we found that potassium per- 

 manganate was the most suitable, since it was 

 exhausted after a period of time and the treated 

 tank did not need to be flushed, as was required 

 with the other chemicals. 



Chilodon spp. 



Only a few outbreaks of disease were attribut- 

 able to this protozoan, a common ectoparasite of 

 warm-water fishes. Davis (1953) states that this 

 organism may be very destructive to fish crowded 

 in small holding tanks or ponds and has been 

 known to cause serious losses among trout 

 fingerlings. 



On a number of occasions, both adult and young 

 tilapia were found to be infested with this para- 

 site. Areas infested were usually the dorsal 

 and caudal fins and along the doi-sal surface of 

 the fish, especially near the base of the dorsal fins. 

 Very frequently, trichodinids were also present. 



Treatment to rid the fish of this parasite was 

 usually with potassium permanganate or with 

 PMA, although the former was used more ex- 

 tensively because of its ease of application. 



Infectious Pancreatic Necrosis 



Infectious pancreatic necrosis, until recently 

 known as acute catarrhal enteritis (Lagler, 1956; 

 Snieszko and Wolf, 1958), was the most serious 



